Surgical tool

A medical instrument for robotic surgery, the medical instrument comprising:

• a frame;
• a shaft having a first end and a second end, wherein the first end of the shaft is attached to the frame;
• a jointed device coupled to the second end of the shaft, the jointed device comprising;
• a first link;
• a second link connected to the first link by a first rotational joint;
• a third link connected to the second link by a second rotational joint and comprising a wrist member having a pitch axis of joint movement and a yaw axis of joint movement;
• and
• a fourth link connected to the third link by a third rotational joint;
• and
• two pairs of tendons extending from the frame, through the shaft, to connect with the joint device,
• wherein at least one of the two pairs of tendons is attached to the fourth link and is configured to move the fourth link with respect to the third link by pulling the on fourth link,
• wherein each tendon of the two pairs of tendons comprises;
• a first tendon endpoint attached to the frame;
• a second tendon endpoint attached to at least one of the second link or the third link;
• and
• a main portion, extending between the first tendon endpoint and the second tendon endpoint;
• wherein each of the first link, the second link and the third link comprises a main structural body with a convex, ruled surface, and
• wherein the main portion of each tendon of the two pairs of tendons contacts the jointed device only on the convex, ruled surfaces of the first link, the second link and the third link.